Does interleukin-6 signaling drive sickness-related fatigue and pain hypersensitivity?

Interleukin-6 (IL-6) acts as a pivotal immune-to-brain signaling molecule that coordinates both behavioral responses to infection and the amplification of pain processing within the central nervous system.

Clinical evidence for fatigue and pain

• Sickness behavior and fatigue: Longitudinal data from the Baltimore Longitudinal Study of Aging demonstrates that chronically elevated IL-6 levels independently correlate with increased perceived fatigability in adults. While IL-6 is mechanistically linked to hypoactivity and anxiety-like behaviors in preclinical models, clinical blockade of IL-6 (e.g., tocilizumab) has shown mixed results. In some populations, such as hematopoietic cell transplant patients, inhibiting IL-6 signaling has paradoxically exacerbated depression and sleep disturbances, suggesting the cytokine may play a complex, non-linear role in mood and sleep regulation.
• Pain hypersensitivity: Clinical genetic data indicates that individuals with the IL-6 rs1800795 GG genotype, which is associated with higher IL-6 production, require significantly higher doses of postoperative opioids, reflecting enhanced pain sensitivity.

Mechanistic explanations

• Central sensitization: IL-6 facilitates central sensitization by shifting the excitatory-inhibitory balance in dorsal horn neurons. It reduces the frequency of inhibitory postsynaptic currents (sIPSCs) while simultaneously enhancing excitatory currents (sEPSCs).
• NMDA potentiation and plasticity: IL-6 potentiates NMDA receptor-induced currents, leading to increased calcium influx and glutamatergic signaling. This process activates the JAK/STAT3 pathway and induces the phosphorylation of CREB in the superficial dorsal horn, driving long-term synaptic plasticity (LTP) and sustaining tactile allodynia.
• Immune-to-brain signaling: IL-6 mediates sickness behavior by interacting with the hypothalamus and limbic structures, often following acute immune challenges like lipopolysaccharide (LPS) administration, which triggers systemic inflammatory cascades.

Bottom line

IL-6 is a validated driver of central sensitization and pain hypersensitivity via NMDA potentiation and dorsal horn hyperexcitability. While it is fundamentally linked to sickness-induced fatigue, therapeutic targeting requires caution as it may involve essential regulatory functions for mood and sleep.